There have been known techniques for optically measuring blood amount variation in and around a surface layer of a brain of a person by irradiating near infrared light to the head of the person. The known blood amount measuring techniques are based on detection of blood distribution performed by detecting a state of existence of hemoglobin utilizing a difference between light absorbing characteristics of oxygenated hemoglobin and deoxygenated hemoglobin. In the near infrared light measurement by the known optical measuring apparatus based on near infrared light irradiation, a measuring probe unit, which includes a plurality of optical fibers secured to a flexible base plate, is attached to the head of a person to be measured. Near infrared light is irradiated via the optical fibers to the head, and then an analysis is made of diffuse reflected light having passed through a surface layer of the brain and its vicinity. The analysis is intended to identify a blood distribution state in the measured portion of the brain surface layer, to thereby identify active areas of the brain changing during the optical measurement.
An example of the apparatus for performing the above-mentioned optical measurement based on near infrared light irradiation is disclosed in Japanese Patent Application Laid-Open Publication No. HEI-11-164826. The disclosed optical measuring apparatus includes a photo detector for converting, into electricity, light having passed into and then got out of a desired portion of an object to be measured (i.e., to-be-measured object or test subject), and a light detection section including a circuit for amplifying the converted electrical signal or a circuit for distinguishing a given frequency component. The disclosed optical measuring apparatus also includes a bias adjustment circuit and amplifier within the light detection section or at a stage following the light detection section. Signal adjusting value of the bias adjustment circuit and gain of the amplifier are set in accordance with intensity of the passed light and noise contained in the passed light.
Another example of the optical measuring apparatus is disclosed in Japanese Patent Application Laid-Open Publication No. HEI-11-169361, which irradiates light to an object to be measured and acquire information indicative of a status within the to-be-measured object. In the disclosed apparatus, light emitted by a light source, whose intensity can be modified at a desired frequency, is irradiated to the object, and a component of the irradiated light having passed into and then got out of the object is detected and converted into an electrical signal. The converted signal is then passed through a frequency filter, amplified and then subjected to phase detection.
The conventionally-known optical measuring apparatus would, however, present the following two problems.
First Problem:
Where the head of a person is to be optically measured or investigated by use of any one of the above-mentioned optical measuring apparatus utilizing near infrared light, a measuring probe unit is attached to and fixed to the head of the to-be-measured person. In this case, it is necessary to push aside the head hair with a rod-shaped implement and then firmly press respective distal ends of a plurality of optical fibers, provided on the measuring probe unit, into close contact with the skin of the head. Pushing aside the hair is necessary to allow near infrared light to be appropriately irradiated through the optical fibers to the surface of the head skin without being hindered by the hair. The conventional measuring probe unit is constructed to permit “24-point measurement”, where the near infrared light is irradiated to eight points or spots of the head skin. For that purpose, there are provided eight light irradiation sections and eight light detection sections on a substantial square area of the probe unit opposed to the head skin.
In the case where such a 24-point measuring probe unit is used, even a skilled human operator would ordinarily take about 20 minutes of preliminary arrangements (preparations) for securing the probe unit to the head of the to-be-measured person while meeting the aforementioned requirements. Further, in fixing the measuring probe unit to the person's head, there is a need to press the inner surface of the probe unit into close contact with the head skin. Thus pressing the inner surface of the probe unit into close contact with the head skin, however, imparts considerable pressure to the head, so that the to-be-measured person may feel a pain in the head and have a uncomfortable feeling during the optical measurement. Further, with the conventional optical measuring apparatus where the measuring probe unit is pressed to closely contact the head skin of the to-be-measured person, the probe unit and hence the distal ends of the optical fibers provided thereon tend to be undesirably displaced as the to-be-measured person moves his or her head. Therefore, the spot of the head skin where the irradiated near infrared light enters (i.e., incident position) and the position where scattering reflected light from the head skin is detected (i.e., reflected light detection position) can not be kept constant, which would adversely influence results of the optical measurement.
Second Problem:
Referring to FIG. 18, if the near infrared light 202 is irradiated from the light irradiation section 200 to the head 201, reflection, refractive transmission and scattering of the irradiated near infrared light 202 are repeated between the head skin 203, skull 204 and cerebrum 205. Then, reflected light 206 due to diffusion and scattering (hereinafter, diffuse/scattering reflected light), having got out of the head 201, is detected by a light detection mechanism 207. Because a plurality of arteries exist in the head skin 203 and its vicinity, the reflected light 206 detected by the optical measuring apparatus would be unavoidably influenced by the arteries 208. For example, where blood distribution in the cerebrum is to be measured, and if the pulsation of the arteries 208 influences, a signal produced due to blood flows in the arteries 208 undesirably overlaps a light detection signal (measurement signal) representative of detected reflected light from the head, so that accurate measurement of the blood distribution in the cerebrum 205 tends to be difficult to achieve.